Ytterbium: uses

The following uses for ytterbium are gathered from a number of sources as well as from anecdotal comments. I'd be delighted to receive corrections as well as additional referenced uses (please use the feedback mechanism to add uses).

possible use in improving the grain refinement, strength, and other mechanical properties of stainless steel

one isotope apparently used as a radiation source as a substitute for a portable X-ray machine where electricity is unavailable